Tropical Cyclone Rainfall David Roth NOAA Weather Prediction Center College Park, MD Last Updated: April 30, 2015

Role in the Tropical Cyclone Program Collaborative track forecast – medium range Rainfall Statements composed by WPC WPC assumes responsibility for inland depressions outside of Florida Service Backup for NHC if needed

Medium Range Collaboration on days 3-7 forecast for systems not yet formed, or days 6-7 for active tropical cyclones.

Rainfall Statement Expected areal average rainfall and isolated maximum amounts Atlantic and eastern Pacific basins …total rain accumulations between 4 and 6 inches expected, and isolated totals of 12 inches possible. Heavy rain could potentially affect the other islands Saturday and Sunday. This rainfall could cause life-threatening flash floods and mud slides.

Public Advisories – Inland Inland advisories for cyclones that have been downgraded to tropical depression – outside Florida Forecast positions to Day 5 or loss of definable center Discontinued if system is no longer a flash flood threat Content: Current Watches/Warnings/Advisories Storm description / intensity Observed rainfall summary Forecast evolution and track

Deterministic QPF WPC RFCs WFOs 6-hourly to 84 hours 48-hour summation days 4/5, 6/7 Graphical, gridded, kml Associated discussions Manual product drawn at 20-km Downscaled to 5km WPC RFCs WFOs 6

Tropical Cyclone Rainfall Graphic WPC, NHC home page during tropical cyclones… Variable duration summation – storm dependent Media visibility Note: Color scale and minimum contour of 2” was specified by FEMA. Alternate versions are now being considered.

Probabilistic QPF When the manual WPC forecast is ready, it is lumped with the NAM, GFS, ECMWF, and SREF to form a 25 member ensemble. The probabilistic distribution is then forced into this format: WPC deterministic QPF is the mode (most likely value), variance is that of the ensemble, and skew is based on the position of the WPC manual forecast WPC is the “most likely” deterministic value Probability QPF 8

Probabilistic QPF View 6- or 24-hourly QPF by Percentile Probability of QPF Exceeding a Threshold Great decision support tool. We’re forecasting 2 inches, but there’s a small chance you could receive 5 inches. View 6- or 24-hourly PQPF and PWPF updates lag deterministic products by

Flow of rainfall-related data to and from the Weather Prediction Center (WPC)

Flow of rainfall data to WPC

Flow of tropical cyclone rainfall forecasts within the United States National Weather Service (NWS) Statement Four times a day Graphics Four times A day

Tropical Cyclone Rainfall Climatology

Timing of Peak activity in Tropical Cyclone Basins After Gray (1975) /Dr. J. Marshall Shepherd (University of Georgia/NASA)

Characteristics of TC precipitation Stratiform and Convective mechanisms Stratiform rain ~50% of total rain from TC. WSR-88D DPA daily accumulations Hurricane Irene (15 October 1999) Frank Marks (HRD)

TC rainfall peaks when global rainfall is low Asymmetric-generally more rain in the Northern Hemisphere Global rainfall is decreasing with increasing latitude while TC rainfall is increasing TC contributes 10-17% of global rain 15-30° poleward from Equator (subtropics) TC Rain Frank Marks (HRD)

United States Units in cm 2005 US Summer rain 2005 US TC rain Frank Marks (HRD)

Tropical Cyclone vs. Total Seasonal Rainfall in Mexico Art Douglas (Creighton University)

Biggest Rain Producers by Country/Island Anguilla 490.0 mm 19.29” Lenny (1999) Belize 829.8 mm 32.67” Keith (2000) Bermuda 186.7 mm 7.35” October 1939 Hurricane Canada 302.0 mm 11.89” Harvey (1999) Cayman Islands 764.8 mm 31.29” Sanibel Island Hurricane (1944) Cuba 2550 mm 100.39” Flora (1963) Dominica 422.3 mm 16.63” Jeanne (2004) Dominican Rep. 1001.5 mm 39.43” Flora (1963) Guadeloupe 508 mm 20.00” Marilyn (1995) Haiti 1447.8 mm 57.00” Flora (1963) Honduras 912 mm 35.89” Mitch (1998) Jamaica 2451 mm 96.50” November 1909 Hurricane Martinique 680.7 mm 26.80” Dorothy (1970) Mexico 1576 mm 62.05” Wilma (2005) Nicaragua 1597 mm 62.87” Mitch (1998) Panama 695 mm 27.36” Mitch (1998) Puerto Rico 1058.7 mm 41.68” T.D. #19 (1970) St. Martin/Maarten 866.6 mm 34.12” Lenny (1999) Swan Islands 362.7 mm 14.28” Alma (1966) United States 1219 mm 48.00” Amelia (1978) Venezuela 339 mm 13.35” Bret (1993)

United States

Mexico

Percent of Maximum storm total rainfall (Hrs) 81 cases – 1991-2005 Average Maximum 0.92 1.40 1.76 2.04 2.98 3.61 5.01 6.71 9.77 11.48 12.34 13.34 1.90 3.04 4.56 6.08 10.04 13.47 22.27 28.33 32.52 35.29 36.31 40.68

Percent of Wettest Southwestern TC Rainfall Per Time Frame (hours) 1992-2006 (8 cases) Average .56 .94 1.10 1.26 1.70 2.00 2.83 3.15 4.64 5.02 5.05 Max. .90 1.60 2.20 2.60 3.10 3.20 4.00 6.60 10.24 12.01 12.01

Tropical Cyclone QPF

Factors impacting rainfall distributions in landfalling TC’s Storm track (location) Time of day – core rainfall overnight/ outer band rainfall during day Storm size (positive) – the bigger the storm, the more it rains at any given spot Topography – Positive in the upslope areas, but negative past the spine of the mountains Wind shear (negative) – leads to a quicker dropoff in rainfall for inland TCs Nearby synoptic-scale features/Extratropical Transition

TC Model Track Error (km) (2002-2006)

Time of Day – Alberto, July 4-5, 1994 04/18z 00z 05/06z 12z 18z

Storm Size <2 degrees “Very small/ midget” Charley 2-3 degrees Determined by distance from center to outermost closed isobar <2 degrees “Very small/ midget” Charley 2-3 degrees “Small” Allison 3-6 degrees “Average” Frances 6-8 degrees “Large” Wilma >8 degrees “Very large” Gilbert Joint Typhoon Warning Center

Sizes of country/island vs. tropical cyclone United States 9,800,000 square km Mexico 2,000,000 square km Tropical Cyclone 350,000 square km Cuba 111,000 square km Hispaniola 76,500 square km Puerto Rico 9,100 square km Martinique 1,130 square km Dominica 750 square km St. Lucia 620 square km Barbados 430 square km Grenada 340 square km St. Maarten/Martin 87 square km In countries and island groups which are significantly smaller than a TC, chances are the island group will not see the maximum rainfall within the tropical cyclone as it would have sampled a small percentage of the system. In larger countries like the United States, the TC moves within the borders and its rainfall distribution is well-sampled. It is best to regard maximum amounts one would expect with a TC impacting a larger country as a theoretical maximum rainfall amount for smaller islands.

How Mountains Affect the Precipitation Distribution HPC uses the PRISM climatology as a guide for forecasting rainfall maxima in mountain areas. PRISM data is available for every month of the year for the continental United States. http://www.prism.oregonstate.edu/index.phtml

Size and Topography Hurricane Frances (2004)

Size and Topography Hurricane Juliette (2001)

Vertical Wind Shear Heaviest rain tends to fall left and downwind of the shear vector. If the shear is strong enough, all rainfall may move away from the center (exposed center) Shear vector

Depth of Upper Trough Causing Recurvature Key Storms which drop most of the rain right of track are steered predominantly by shear lines or through a break in the subtropical ridge. Rainfall tends to be concentrated near and right of track. Storms which drop most of their rain left of track recurve due to significant upper troughs in the Westerlies. Rainfall streaks out well to the north of the system due to jet streaks moving around the upper trough and frontogenesis at the trough’s leading edge.

Bertha Norman W. “Wes” Junker

Floyd Norman W. “Wes” Junker

Bertha (1996) vs. Floyd (1999)

Bosart and Carr (1978) conceptual model of antecedent rainfall PRE STATISTICS Separation Distance 1086 ± 482 km Median: 935 km Event Duration 14 ± 7 h Median: 12 h Time Lag 45 ± 29 h Median: 36 h Bosart and Carr (1978) conceptual model of antecedent rainfall

PRE TRACK-RELATIVE POSITIONS Potential for flooding in areas not directly impacted by TC rainfall 26 12 9

PRE TRACK-RELATIVE POSITIONS Potential for excessive flooding beginning before arrival of TC rainfall 26 12 9

PRE TRACK-RELATIVE POSITIONS 24-h rainfall rate statistics (mm) Type of PRE (Number in category) 24-h rainfall rate statistics (mm) Mean PRE speed (m s−1) Mean Std. deviation Maximum Left of Track (22) 185 70 340 10.7 Along Track (8) 245 100 410 12.9 Right of Track (7) 260 80 5.7 GREATEST RAINFALL SLOWEST MOVEMENT

PRE TRACK-RELATIVE POSITIONS 24-h rainfall rate statistics (mm) Type of PRE (Number in category) 24-h rainfall rate statistics (mm) Mean PRE speed (m s−1) Mean Std. deviation Maximum Left of Track (22) 185 70 340 10.7 Along Track (8) 245 100 410 12.9 Right of Track (7) 260 80 5.7 HIGH RAINFALL PREs MOVE TWICE AS FAST

2100 UTC 060830 700 hPa Ht (dam) and WSI NOWRAD image Along Track PREs 2100 UTC 060830 700 hPa Ht (dam) and WSI NOWRAD image 2100 UTC 060830 925 hPa Ht (dam), θe (K), and 200 hPa wind speed (m s-1) Ernesto (2006) NW/SE oriented trough well to the northeast Closed midlevel low NW and flat ridge east of TC Broad upper-level jet to the north On western edge of θe ridge

0900 UTC 050830 700 hPa Ht (dam) and WSI NOWRAD image Right Of Track PREs 0900 UTC 050830 700 hPa Ht (dam) and WSI NOWRAD image 0900 UTC 050830 925 hPa Ht (dam), θe (K), and 200 hPa wind speed (m s-1) Katrina (2005) Large midlevel low NNE and ridge SE of TC PREs a bit downstream of where model predicts Jet dynamics only partially explain the PREs No prominent low-level θe ridge or gradient near PRE

0000 UTC 050707 700 hPa Ht (dam) and WSI NOWRAD image Null Case 0000 UTC 050707 700 hPa Ht (dam) and WSI NOWRAD image 0000 UTC 050707 925 hPa Ht (dam), θe (K), and 200 hPa wind speed (m s-1) Cindy (2005) WNW flow at midlevels Scattered rainfall over New England not related to Cindy Massive low-level ridge poleward of TC No rainfall near low-level θe ridge

Rainfall forecasts from landfalling TC’s standard forecasting tools Empirical Methods In-house Tropical Cyclone Rainfall Climatology http://www.hpc.ncep.noaa.gov/tropical/rain/tcrainfall.html GFS/NAM/GFDL/WRF precipitation forecasts r-CLIPER (Climatology based on 1st order stations) TRaP (persistence to capture structure/Day 1) standard validation tools bias score equitable threat score

Rules of Thumb Kraft Rule – 1950’s guideline based on a broad grid of first order sites. Will not indicate the maximum in most cases (R=100/forward motion in knots). Environment Canada/Canadian Hurricane Center use a modified version of Kraft which halves this amount since most systems entering the country are sheared or moving over cooler water prior to landfall. 16-inch rule – Long term average of tropical cyclone rainfall maxima which strike the United States. Vertical wind shear, small sized tropical cyclones, or movement over cooler water prior to landfall can individually lead to a reduction of about half of this figure. Slow moving and larger than average tropical cyclones lead to higher values than the average.

Derived Equation used to Determine TC Rainfall Maxima Rainfall maximum=2*(radial velocity)*(specific humidity in subcloud layer)*(pressure difference within lower km of atmosphere)*(1/radius from center)*(1/g) Riehl (1954) Within 30 nm of center – 863 mm/33.98” per day Within 60 nm of center – 160 mm/ 6.30” per day Within 120 miles of center – 15 mm /0.59” per day Assumes a symmetric/non-sheared hurricane with a gale radius around two degrees of latitude/120 nm. Does not take into account topography or nearby frontal zones.

Old TAFB Method RAIN ACCUMULATION DIAMETER * RAIN RATE VELOCITY = Convective Rainfall Rates Average Climatological Rain Rate = 2 mm / hour Or 0.08 in./hour Core Rain Rate = 5 times this Average or Core Rain Rate = 10 mm /hour Or about 0.40 in./hour Reinforced by radial amounts computed within Jiang, Halverson, Simpson AMS Hurricane Conference preprint (2006)

RAINFALL CALCULATION USING UNENHANCED INFRARED IMAGERY Storm Name: ___________________ Date: ________________ 19__ Image Date/Time Diameter of Storm in Direction of Motion ____________ UTC _________ deg * 110 km/deg = _________ km Mean Diameter: D = ________ km FREDERIC 12 SEPT 79 12 / 0630 5.5 605 12 / 1200 5.5 605 12 / 1800 4.0 440 12 / 0000 4.5 495 540

TROPICAL CYCLONE RAINFALL ESTIMATION HURRICANE FREDERIC, SEPTEMBER 1979 6-HOUR CONTINUITY, INFRARED

Forecast translation speed: V = _____ deg Forecast translation speed: V = _____ deg * 110 km/deg / 18 hrs = _____ km/hr Mean rainfall rate: R = 0.2 cm/hr D * R Rainfall Potential: P = ------- V km * 0.2 cm/hr P = ------------------------------------ = ________ cm km/hr Core Rainfall: C = 5 * P = ________ cm 450 450 Rule of Thumb: T = ---------------- = ----------------- = ________ cm V km/hr km/hr 4.0 24 540 4.5 24 22.5 (8.9”) 18.8 (7.4”) 24

Frederic Rainfall

3 event-driven products Eastern Caribbean (40°W to 67°W) TCCA21 KNHC NHC Satellite Tropical Disturbance Rainfall Estimates 3 event-driven products Eastern Caribbean (40°W to 67°W) TCCA21 KNHC MIASTDECA Central Caribbean (67°W to 80°W) TCCA22 KNHC MIASTDCCA Western Caribbean/Mexico (80°W to 120°W) TCCA23 KNHC MIASTDWCA

Picking an analog for a TC event Size is important…look at the current rain shield and compare it to storm totals/storms from the past How fast is it moving? Vertical wind shear in current/past events? Look for storms with similar/parallel tracks Is topography/prism data a consideration? Look for nearby fronts/depth of nearby upper troughs for current and possible analogs Not all TC events will have a useful analog

CLIQR Scripts utilize extended best track database from NHC, modified by additional information from HPC/NHC map series and NHC Atlantic non-developing system database Storm matches made primarily upon current position, forward motion, and storm size. In 2009, NHC five day track will be included Uses a 9 point system. The system’s point total can be seen in the last column of text output Output generated using CHGHUR/objective guidance messages from NHC, but can also be utilized using manual input Simplified output online for active systems at: http://www.hpc.ncep.noaa.gov/tropical/rain/web/cliqr.html

CLIQR GUI using manual input If there is a current system, the CHGHUR identifier shows up within the Select Storm window.

CLIQR output via magenta “View Rainfall Graphics” button The .gif comes from my account on the local network. More Info button calls up the appropriate web page from the Tropical Cyclone Rainfall Project, also from within the HPC network.

CLIQR web output for active systems http://www.wpc.ncep.noaa.gov/tropical/rain/web/cliqr.html

CLIQR matching storm list (Rainfall Matches hyperlink) Simplified list links to relevant storm total rainfall graphic through hyperlink. Future revisions include columns separating the Puerto Rico impacts from North American impacts to help web user.

Isbell (1964) vs. Wilma (2005)

Isabel (2003) vs. Fran (1996)

Example A Tropical Storm has formed east of the Leeward Islands. It is expected to move west-northwest near 12 knots over the next 5 days, becoming a hurricane prior to reaching Florida, and it is smaller than average in size. Wind shear is expected to pick up out of the northeast in a couple days. What is the rain potential for Puerto Rico and the Dominican Republic?

Elevation of Hispaniola/Puerto Rico

Considerations Factors leading to higher rainfall Factors leading to lower rainfall Convection should be concentrated in its southern quadrant due to wind shear Significant topography in Dominican Republic and Puerto Rico Smaller than average Vertical wind shear is expected to increase Moving greater than 6 knots Core of system expected to pass north of the islands

Result System moved closer than expected to Dominican Republic and dissipated as it reached Cuba (Chris 2006)

Example An average sized category 2 hurricane is moving towards the Gulf coastal plain of the United States. The forecast is for the storm to recurve at 3 knots into the Deep South with no wind shear anticipated.

Considerations Factors leading to higher rainfall Factors leading to lower rainfall No wind shear Movement under 6 knots Tropical cyclone moving into a larger landmass Could be an area within the track’s radius of curvature which receives training None

Result System well-forecasted, so a wetter than average impact was well-anticipated (though initially underforecast)

Production of TC QPF Forecasts made in six-hourly increments from Hour 6-84 and in two 48 hour chunks for Hours 84-132 and 132-180 twice a day by 3 forecasters (Day 1, Day 2/3, and Medium Range temps/pops) Start With Model Closest to TPC Forecast (usually GFS) Locate relevant synoptic scale boundaries/coastal front Use conceptual models/current structure to modify/shift QPF (TRaP and recent satellite/radar imagery for current structure) Look at storm-relative shear/H2 winds to further shift/limit QPF Use climatology (PRISM, r-CLIPER, TC Rainfall Climatology) to: Temper down forecast bias/act as a reality check Depict areas of terrain that could be significantly impacted Help Create TC rainfall statements for the Public Advisories Forecasts issued at by 06/18z (Days 1-3) and 12z/0z (Days 4-5, 6-7 and 7-day accumulation graphic)

Dependence on TPC track - Rita Threat/Bias for 5 Day QPF September 21/12z Forecast 0.25 .453 1.52 H G .498 1.39 0.50 .350 1.46 H G .414 1.35 1.00 .197 .961 H G .258 1.24 2.00 .030 .725 H G .168 .858 3.00 .013 1.28 H G .093 1.06 4.00 .009 2.61 H G .069 1.86 5.00 .000 3.49 H G .021 3.01 6.00 .000 4.23 H G .018 4.69 September 22/12z Forecast 0.25 .536 1.33 H G .541 1.08 0.50 .468 1.18 H G .534 .978 1.00 .367 1.07 H G .366 .781 2.00 .164 .777 H G .234 .792 3.00 .163 1.35 H G .224 .916 4.00 .128 2.50 H G .199 1.63 5.00 .090 3.74 H G .174 2.18 6.00 .090 5.71 H G .161 2.98

Specialized Tropical Cyclone QPF Guidance

R-CLIPER R-CLIPER (Rainfall Climatology Parametric Model) Statistical model developed from TMI data and rain gauges Simple model creates a rainfall swath dependent on storm track, intensity, and size Operational at 0.25o X 0.25o hourly resolution Asymmetries are not taken into account

R-CLIPER Improvements Includes shear and topographic effects in 2007 R-CLIPER + Shear + Topog R-CLIPER R-CLIPER + Shear

TRaP Can be found at NOAA/NESDIS Satellite Analysis Branch (SAB) http://www.ssd.noaa.gov/PS/TROP/trap.html Uses microwave rain rate images from SSM/I, TRMM, and AMSU and extrapolates along TC forecast track. METOP and SSMI/S, part of AMSU, expected by the end of the year Only available when a microwave pass “catches” the storm mostly within the swath Depends on official forecast of TC track from NHC, CPHC, etc.

TRaP: http://www.ssd.noaa.gov/PS/TROP/trap.html

Katrina Rainfall

WPC tropical cyclone QPF verification 2004-2008

Day 1 Threat Scores and Bias

Day 2 Threat Score/Bias

Day 3 Threat Score/Bias

Summary Tropical cyclones lead to 10-20% of annual rainfall in South and Eastern U.S. While tropical cyclones lead to a smaller percent of annual rainfall for the Desert Southwest regionally on an annual basis, individual events can lead to a significant portion of the annual rainfall on a local basis. Tropical cyclone QPF pattern depends on storm size, forecast track, vertical wind shear, topography, depth of upper trough causing recurvature, and SST field the cyclone moves over prior to landfall While climatology is important to keep in mind, TC QPF is heavily based on the guidance which has the best verification and is closest to expected TC track (usually GFS). NAM and ECMWF both show low biases for higher rainfall amounts.

National Forecst Chart Summary WPC has the tools and focused mission to add value to the raw model output and frame local issues in a big-picture context Ramping up attention to the mesoscale… facilitating a collaborative process Very attentive to tropical cyclones, as they influence all of our products Historical Rainfall Archive National Forecst Chart WPC/NHC/OPC Unified Analysis